As is known, tensioners for accessory drives comprise a fixed part configured to be fastened to the engine and defining a rotational pivot, an arm rotatably supported on the rotational pivot and carrying, at one end, a pulley suitable for cooperating with a drive belt and a spring acting on the arm to tension the belt.
Known tensioners of the above-stated type must meet certain functional requirements aimed at optimizing the life of the tensioner and of the belt. In particular, it is necessary that the arm is kept in perfect alignment, i.e. it rotates on a plane orthogonal to the axis of the pivot and is not subjected to oscillations outside the plane that might result in noise, premature wear and, eventually, belt slippage.
The alignment function is normally delegated to one or more bushings, made of a plastic or composite material, having the function of a sliding bearing for the radial and axial support of the arm on the pivot.
These bushings are subject to wear; in consequence, the tensioner has a limited life, in any case shorter than that of the vehicle.
Hydraulic tensioners have been developed to solve this problem, wherein the force acting on the arm is generated by a hydraulic piston. Since the piston can be oriented to minimize the resultant of the forces acting on the pivot, hydraulic tensioners are normally components not subject to replacement during the life of the vehicle. However, hydraulic tensioners are more complex and expensive than conventional mechanical tensioners.